The present invention relates generally to forming permanent magnets for use in electric motors, and more particularly to including rare earth (RE) materials to improve magnetic properties of the formed magnets, as well as to use high-velocity compression techniques as a way to form magnets into shapes that require little or no post-formation machining.
Permanent magnets have been widely used in a variety of devices, including traction electric motors for hybrid and electric vehicles, wind mills, air conditioners and other mechanized equipment. One type of permanent magnet—sintered Nd—Fe—B type permanent magnets—contains RE metals such as dysprosium (Dy) or terbium (Tb) to improve the magnetic properties (such as intrinsic coercivity) of the magnets at high temperatures.
Known RE magnet manufacturing processes begin with the initial preparation, including inspection and weighing of the starting materials (iron, iron-neodymium alloy and boron, as well as iron-dysprosium alloys or the like) for the desired material compositions. The materials are then vacuum induction melted and strip cast to form thin pieces (less than one mm) of several centimeters in size. This is followed by hydrogen decrepitation where the thin pieces absorb hydrogen at about 25° C. to about 300° C. for about 5 to about 20 hours, dehydrogenated at about 200° C. to about 400° C. for about 3 to about 25 hours and then subjected to hammer milling and grinding and/or mechanical pulverization or nitrogen milling (if needed) to form fine powder suitable for further powder metallurgy processing. This powder is typically screened for size classification and then mixed with other alloying powders for the final desired magnetic material composition, along with binders to make green parts (typically in the form of a cube) through a suitable pressing operation in a die (often at room temperature). In one form, the powder is weighed prior to its formation into a cubic block or other shape. The shaped part is then vacuum bagged and subjected to isostatic pressing, after which it is sintered (for example, at about 900° C. to about 1100° C. for about 1 to about 30 hrs in vacuum) and aged, if needed, (for example, at about 300° C. to about 700° C. for about 5 to about 20 hours in vacuum). Typically, a number of blocks totaling about 300 kg to about 500 kg undergo sintering at the same time as a batch. The magnet pieces are then cut and machined to the final shape from the larger block based on the desired final shape for the magnets. The magnet pieces are then surface treated, if desired.
Normally with the powder metal process, the density of the green part is about 50 to 55 percent of the theoretical density, which results in significant shrinkage during sintering. If the green part is in cubic block form, the shrinkage is uniform. However, if the green part is not symmetric in shape, it will distort and warp in a manner that is typically difficult to control. To avoid this, the required magnets are usually machined from the block material; this process results in a relatively large amount of material loss, where the yield is typically about 55 to 65 percent (i.e., about 35 to 45 percent loss of the material). Other difficulties associated with the conventional powder metallurgy-based technique also arise. For example, the surfaces of the original large block are also subject to some oxidation, which may result in additional loss of material.
The high material loss during manufacturing has greatly increased the cost of the finished RE magnets. This cost has been exacerbated by a dramatic rise in the price of the raw RE metals in the past several years. As such, there are significant problems associated with accurately producing cost-effective magnets that contain RE materials.